With a construction vehicle such as, for example, a wheel loader or the like, when performing a task such as excavation that requires a large travel drive force, if the travel drive force being outputted from the travel drive wheels (i.e. the travel propulsion force) is excessively great in view of the state of the road surface, slippage between the tires and the road surface, destruction of a fragile road surface, or the like may occur, and this leads to a decrease in the efficiency of working. Moreover, tire slippage is accompanied by early wear and tear upon the tires, and this leads to the tire exchange frequency becoming high and to increase of the cost of vehicle maintenance.
If, in order to solve this problem, the driver observes the state of the road surface and sets the travel drive force by a manual setting procedure such as by operating a dial or the like, then, when the travel drive force that is actually being outputted during excavation (i.e. the actual drive force) exceeds this set drive force, a technique may be implemented by the wheel loader or the like for steadily reducing the actual drive force according to a decrease value that corresponds to this deviation between the actual drive force and the set drive force.
Moreover, techniques are also known for presenting slippage by detecting a symptom of slippage of the travel drive wheels and by adjusting the degree of engagement of a modulation clutch, or by adjusting the fuel injection amount for the engine (for example, refer to Patent Documents #1 and #2).